Parallel flow diffusion battery

ABSTRACT

A parallel flow diffusion battery for determining the mass distribution of an aerosol has a plurality of diffusion cells mounted in parallel to an aerosol stream, each diffusion cell including a stack of mesh wire screens of different density.

BACKGROUND OF THE INVENTION

The present invention relates generally to an aerosol diffusion batteryand, more particularly, to a parallel flow, screen-type diffusionbattery. The U.S. Government has rights in this invention pursuant toContract Number DE-AC04-76EV01013 between the U.S. Government andLovelace Biomedical and Environmental Research Institute, Inc.

A diffusion battery is a useful and reliable instrument forcharacterizing aerosol particles of less than 0.1 micrometer (μm) indiameter. The diffusion battery directly provides the diffusionequivalent diameter (equivalent to a spherical particle with the samediffusion coefficient) of irregularly shaped particles. The conventionaldiffusion battery consists of several stages of diffusion cellsconnected serially, with each cell comprising either a collimated holestructure containing many parallel holes bundled together or a stack ofwire screens. When an aerosol is passed through a diffusion cell, aportion of the aerosol diffuses to the tube wall or the wire surface andis collected there. Therefore, the aerosol concentration decreasescontinuously as it goes through the successive diffusion cells. Bymeasuring aerosol concentrations at each successive stage of thediffusion battery, the fractional penetration of an aerosol can bedetermined. Since fractional aerosol penetration depends upon aerosolsize, the size distribution from the aerosol penetration data may beestimated.

A description of one commercially available screen-type diffusionbattery of the series type (Model 3040, TSI, Inc., St. Paul, Minn.) maybe found in an article by the inventors entitled, "Theory of aScreen-Type Diffusion Battery", J. AEROSOL SCI., Vol. 11, pp. 313-320,1980. This device gives an accurate reading of the particle numberdistribution of an aerosol. However, mathematical conversion of numberdistribution to mass distribution (a reading which is more useful inmany toxicology studies) gives inaccurate results unless the aerosolparticles are spherical.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a diffusion battery whichyields a direct indication of aerosol mass size distribution.

It is another object of this invention to provide a seven-cell, parallelflow, screen-type diffusion battery for the size characterization andsize classification of ultra-fine aerosols.

Additional objects, advantages and novel features of the invention willbecome apparent to those skilled in the art upon examination of thefollowing description or may be learned by practice of the invention.The objects and advantages of the invention may be realized and attainedby means of the instrumentalities and combinations particularly pointedout in the appended claims.

To achieve the foregoing and other objects and in accordance with thepurpose of the present invention, as embodied and broadly describedherein, the diffusion battery of this invention may comprise an intakemanifold for receiving an aerosol and distributing it to a plurality ofdiffusion cells extending through a cell holding surface. Each cell hasa tubular body extending through the surface and means for diffusing anaerosol and a filter serially mounted within the tube. The density ofeach diffusing means differs from the density of the diffusing means foreach other cell, permitting a plurality of measurements to besimultaneously taken from each aerosol sample. In a preferredembodiment, the diffusing means comprises a number of wire screens, thenumber of screens being different within each cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate an embodiment of the present inventionand, together with the description, serve to explain the principles ofthe invention.

FIG. 1 is a partial cutaway of the invention, showing the detail of onediffusion cell.

FIG. 2 is a view of the cell holding surface, showing the position ofthe cells within the invention.

FIG. 3 is a graph showing mass penetration as a function of number ofscreens for a plurality of aerosol sizes.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a partial cutaway view of a parallel flow diffusion batteryin accordance with the preferred embodiment of this invention. Battery 1includes an intake manifold 5, a cell holding surface 10, an outputmanifold 40 and a plurality of diffusion cells 20. For the sake ofclarity, only one diffusion cell 20 is shown in FIG. 1. In accordancewith this invention, it should be understood that there are a pluralityof cells 20 located as shown and discussed hereunder in FIG. 2.

Intake manifold 5 serves as a means for distributing an aerosol evenlyto each of the diffusion cells of the invention. Accordingly, intakemanifold 5 includes a narrow input end 6 where the aerosol under test isfed into the device, a conical portion 7, and a wide output end 8.Intake manifold 5 may assume any configuration which ensures the evendistribution of aerosol to all diffusion cells.

Cell holding surface 10 forms a support for the cells and comprises ametal plate of approximately the same area as the area of wide outputend 8. Surface 10 includes an outer flange 11 extending on either sideof surface 10. End 8 of intake manifold 5 fastens to flange 11 byconventional means such as screw threads 12. O-ring 45 ensures a tightseal, preventing an aerosol leak between manifold 5 and flange 11.

In a similar manner, output manifold 40 has a wide end 41 which issealably fastened to flange 11 by means of threads 43 and O-ring 46.Output manifold 40 also includes an exhaust port 42 which may be used tosafely remove the aerosol from the test environment.

As shown in FIG. 2, cell holding surface 10 provides a barrier betweeninput manifold 5 and output manifold 40, except for a plurality of holes13a-13g. Each hole is filled by one diffusion cell 20.

As shown in FIG. 1, each diffusion cell comprises a tubular body 21having an outer surface 25 of the same outer configuration as the innersurface of hole 13, thereby permitting a tight seal between these parts.O-ring 47 ensures a tight seal between these parts. The inner surface ofbody 21 has a first diameter 22 over a first portion of the bodyextending from the input end and a second smaller diameter 23 over asecond portion of body 21 extending from first portion 22. Step 24connects the two diameters. The output end of body 21 is partiallyclosed by a flow restricting orifice 26.

Aerosol particles are collected for measurement on a filter 35 having adiameter equal to first diameter 22 and resting against step 24. Filter35 is retained in place by spacer 27, a ring having an outer diameterequal to first diameter 22, and an inner diameter approximately equal tosecond diameter 23. The side of spacer 27 opposite filter 35 forms asupport for collimated hole means which, preferably, comprise a stack ofwire screens 30, having an outside diameter equal to first diameter 22.Screens 30 are affixed by removable retainer spacer 28, a cylinderhaving the same diameters as spacer 27 and a length sufficient to extendjust beyond the input end of body 21. Retainer cap 29, having a centeraperture equal to second diameter 23 is threaded onto outer surface 25of body 21 to retain filters 35 and screen 30 in a desirable position.

The principle of operation is as follows. The penetration of an aerosolthrough screen type diffusion battery can be described in terms of slopem as

    m=-(log P/n)=A.sub.o Pe.sup.-2/3 +A.sub.1 R.sup.2 +A.sub.2 Pe.sup.-1/2 R.sup.2/3

where P is aerosol penetration, n is number of screens, Pe=UD_(f) /D isthe Peclet number and R=D_(p) /D_(f) is the intercept parameter. D_(p)and D_(f) are particle diameter and screen wire diameter, respectively.U is the mean flow velocity, D is the diffusion coefficient of theparticle, and A₀, A₁, A₂ are constants depending on the geometry andhydrodynamic parameters of the screen. FIG. 3 shows theoreticalpenetration curves of aerosols from 0.01 to 0.7 μm in the parallel flowdiffusion battery using stainless steel 200 mesh screen and a flow rateof 1.01 liters per minute in each cell. From this theoretical equation,the aerosol size distribution is estimated by comparing the masspenetration data with the theoretical one of FIG. 3. The resultant dataprovides an estimate of aerosol size distribution based on mass, i.e.,mass medium diameter.

In order to provide for a plurality of measurements for a single aerosolsample, each cell 20 has a screen 30 of a different density asdetermined by the number of screens within each cell 20. For example,holes 13a-13g may have cells 20 having 0, 10, 15, 20, 25, 30 and 35screens respectively. Each screen of screen stack 30 can be a 200 mesh,400 mesh or 635 mesh, and the filter 35 is 0.2 μm pore-size Zeflourfilter paper. Orifice 26 of each cell 20 is sized to provide a desiredflow rate of 0.55, 1 or 2 liters per minute operated under the criticalcondition.

The particular components and equipment discussed above are cited merelyto illustrate a particular embodiment of the invention. It iscontemplated that the use of this invention may involve differentcomponents as long as the principle, utilizing a number of paralleldiffusion cells having regular mesh screens of differing densities, isfollowed. A diffusion battery so constructed will provide a sensitive,reliable measurement of the mass distribution of an aerosol. It isintended that the scope of the invention be defined by the claimsappended hereto.

The procedure for a parallel flow diffusion battery measurement is asfollows: First, screens 30 are cleaned and each filter 35 is weighed andrecorded. The screens and filter paper are then assembled in each ofdiffusion cells 20 and the cells are placed within holes 13a-13g of cellholding surface 10. Intake manifold 5 and output manifold 40 are thenconnected and sealed. Inlet 6 is connected to an aerosol source, such asan inhalation exposure chamber or an emission source of interest. Outlet42 is connected to a vacuum pump or a house vacuum source. After asufficient sampling period, battery 1 is disconnected from the sourceand disassembled. Filter paper 35 in each cell is reweighed to determinethe amount of aerosol collected or is counted for radioactivity. Fromthis gravimetric or counting data, the aerosol penetration curve as afunction of the number of screens has been determined.

The flow characteristics of a parallel flow diffusion batter constructedas shown in FIG. 1 and having an inside diameter of output manifold 40of approximately 5 inches was checked by passing CsC1 aerosol taggedwith uranine into a battery having no screens. Fluorometric measurementsof aerosol on the screens of each cell showed the variations of flowrate among cells to be less than 2%.

We claim:
 1. A parallel flow diffusion battery for determining the masssize distribution of an aerosol, said battery comprising:an intakemanifold having input means for input of an aerosol and an output end; acell holding surface extending across and sealing the output end of saidintake manifold; and a plurality of diffusion cells affixed to saidsurface, each cell comprising:a tubular body having an inner surface andan outer surface, said outer surface extending through and sealing ahole in said holding surface; collimated hole means for diffusing anaerosol, said means extending across and sealing the inner surface ofsaid body; and filter means for collecting all aerosol passing throughsaid hole structure; the density of the hole means for each celldiffering from the density of the hole means for each other cell,whereby a plurality of measurements are simultaneously taken from eachaerosol sample.
 2. The parallel flow diffusion battery of claim 1wherein said intake manifold comprises a conical cap having a narrow endforming said input means and a wide end forming said output end.
 3. Theparallel flow diffusion battery of claim 2 wherein the outer surface ofsaid holding surface comprises a flange extending on either side of saidholding surface, said flange forming a seal with the wide end of saidconical cap.
 4. The parallel flow diffusion battery of claim 3 furthercomprising an output manifold having a wide end forming a seal with saidflange and a narrow end for exhaust of said aerosol.
 5. The parallelflow diffusion battery of claim 1 wherein said collimated hole meansconsists of a stack of wire screens, the density for each stack beingvaried by having a different number of screens in each stack.
 6. Theparallel flow diffusion battery of claim 1 including another diffusioncell having no collimated hole means, whereby said cell is used todetermine a reference level.
 7. The parallel flow diffusion battery ofclaim 5 further comprising a flow controlled critical orifice sealingthe output end of said tubular body.
 8. The parallel flow diffusionbattery of claim 6 wherein said tubular body has an input end facingsaid intake manifold and an opposing output end, and said inside surfacedefines a circle of first diameter over a first portion of said bodyextending from said input end and a circle of a second, smaller,diameter over a second portion of said body extending from said firstportion, said inside surface connecting said first and second portionsforming a step; andsaid filter having an outside diameter conforming tothe inside diameter of said first portion, and said filter beingpositioned against said step.
 9. The parallel flow diffusion battery ofclaim 8 further including a hollow spacer ring having an outsidediameter equal to said first diameter and positioned against said filtermeans opposite said step; whereinsaid collimated hole means consists ofa stack of wire screens having an outside diameter equal to said firstdiameter, said stack being positioned against said spacer ring oppositesaid filter, the density for each stack being varied by having adifferent number of screens in each stack; and retainer means forholding said stack against said ring.